(a) Field of the Invention
The present invention relates to an organic electroluminescent (EL) display, a driving method of the EL display and a pixel circuit of the EL display. More specifically, the present invention relates to an organic EL display (OELD), its driving methods and a pixel circuit that can compensate for deviation of the threshold voltage of a thin film transistor (TFT) and achieve high gray scale display when driving the pixels of the OELD by using the TFT.
(b) Description of the Related Art
In general, the OELD electrically excites fluorescent organic compounds to emit light, and performs voltage driving or current driving on a (N×M) number of organic luminescent cells so as to display images. As shown in FIG. 1, the organic luminescent cell comprises an anode (ITO), an organic thin film, and a cathode layer (Metal). The organic thin film comprises multi-layers including an emitting layer (EML), an electron transport layer (ETL), and a hole transport layer (HTL) so as to provide a good balance between the electron concentration and the hole concentration thereby improving emitting efficiency, and it additionally comprises an electron injecting layer (EIL) and a hole injecting layer (HIL).
Methods for driving the organic luminescent cells as described above are categorized as a passive matrix method and an active matrix method. The passive matrix method forms positive electrodes to be perpendicular to negative electrodes and selects lines and drives them, and the active matrix method connects the TFT and a capacitor to each ITO pixel electrode so that the voltage may be maintained according to capacitance.
FIG. 2 shows a conventional pixel circuit for driving an OELD using a TFT, illustrating a pixel from among a (N×M) number of the pixels. Referring to FIG. 2, a current driving transistor (Mb) is coupled to the OELD and supplies current to emit light. The current through the current driving transistor (Mb) is controlled by the data voltage supplied via a switching transistor (Ma). In this instance, a capacitor C for maintaining the supplied voltage during a predetermined frame period is coupled between the source and the gate of the current driving transistor (Mb). The gate of the switching transistor (Ma) responds to an n-th select signal line Select[n], and the source to a data line Data[m].
Referring to FIG. 3, as to an operation of the pixel having the above-described configuration, when the transistor Ma is turned on by the select signal Select[n] supplied to the gate of the switching transistor Ma, the data voltage VDATA is supplied to the gate (node A) of the transistor Mb via a data line. In response to the data voltage VDATA supplied to the gate, the current flows to the OELD via the transistor Mb to emit light.
In this instance, the current that flows to the OELD is expressed as follows:                               I          OELD                =                                            β              2                        ·                                          (                                                      V                    GS                                    -                                      V                    TH                                                  )                            2                                =                                    β              2                        ·                                          (                                                      V                    DD                                    -                                      V                    DATA                                    -                                      V                    TH                                                  )                            2                                                          Equation        ⁢                                  ⁢        1            
where IOELD represents the current flowing to the OELD, VGS represents the voltage between the source and the gate of the transistor Mb, VTH represents the threshold voltage of the transistor Mb, VDATA represents the data voltage, and β represents a constant.
As expressed in Equation 1 and according to the pixel circuit as shown in FIG. 2, the current corresponding to the supplied data voltage VDATA is supplied to the OELD, and in response to the supplied current, the OELD emits light. In this instance, the data voltage VDATA has multi-step values within a predetermined range so as to show the gray.
However, it is difficult for the conventional pixel circuit to achieve high gray scale because the threshold voltage VTH of the TFT varies depending on the manufacturing process thereof. For example, in the case of driving the TFT pixels with data voltage in the range of 3 volts, two data voltages representing adjacent gray levels must be apart from each other approximately by 12 mV(=3V/256) so as to implement 8-bit (256) gray scale. If the deviation of threshold voltage is less than 100 mV, it is difficult to discriminate one data voltage from another which results in decreased gray scale.